Apparatus and method of growing a crystal from a vapor



Dec. 31, 1968 E. N. CURCIO 3,419,417

APPARATUS AND METHOD OF GROWING A CRYSTAL FROM A VAPOR Filed Feb. 1, 1966 REACTIVE GAS W SPENT REACTANT PROTECTIVE GAS EXHAUST TEMP.

INVENTOR. EDMUND N. CURCIO ATTORNEYS United States Patent 3,419,417 APPARATUS AND METHOD OF GROWING A CRYSTAL FROM A VAPOR Edmund N. Curcio, South Orange, N.J., assignor to The Ohio Crankshaft Company, Cleveland, Ohio, a corporation of Ohio Filed Feb. 1, 1966, Ser. No. 524,306 5 Claims. (Cl. 117106) The present invention relates to the art of growing crystals and more particularly to an apparatus and method of growing a crystal from a vapor.

This invention is particularly applicable to epitaxial grain growth, which is generally used in the production of solid state devices, such as diodes and transistors, and it will be described with particular reference thereto; however, it should be appreciated that the invention has much broader applications and may be used in growing crystals of various types from a vapor of a crystal producing material.

During recent years there has been a substantial increase in the production of crystals formed from the solidification of a heated vapor of crystal producing material. Most of the processes used to produce crystals in this manner employ the epitaxial principle wherein a substrate seed of a given crystalline material is subjected to a heated vapor of similar material. The vapor is maintained at a given temperature so that the vapor crystallizes on the seed and grows to form a crystal substantially larger than the seed. In addition, the vapor may produce a junction of the type used in transistors if the vapor is formed onto a crystal having properties different from the vapor. The epitaxial crystal growing principle is used in forming element crystals, such as silicon crystals, as Well as certain compound crystals, such as Group III-V crystals. These grown crystals may be made conductive as n type and/or p type solid state elements according to the type and amount of impurities grown within the crystal.

The primary disadvantage of the epitaxial or vaporization crystal growing process is that it requires a delicate control of the temperature at which the crystal is grown. Without this temperature control, the quality of the crystal is substantially reduced. In addition, production of an inexpensive crystal requires that the growing temperature be reached in a very short time.

Apparatus heretofore used to heat the vapor and hold the vapor at the proper growing temperature have not been completely successful in controlling the temperature of the vapor during crystal growth. Also, the prior apparatus have required considerable time to raise the substrate to the desired growing temperature.

These and other disadvantages have been completely overcome by the present invention which is directed toward an apparatus and method for growing a crystal from a vapor of crystal producing material which method and apparatus accurately controls the growing temperature and raises the substrate to the growing temperature in a very short time.

The normal apparatus for growing a crystal from a vapor of crystal producing material includes a refractory chamber, means for introducing the proper vapor into the chamber and means for controlling the temperature of the chamber to cause the vapor to form a crystal. One aspect of the present invention is directed toward an improvement in this general type of apparatus. In accordance with the invention, the temperature controlling means includes a container having a bath of molten metal, means for maintaining the bath at the desired temperature, and means for moving the chamber between a first position where the chamber is in the bath to heat the chamber to the proper growing temperature and hold the 3,419,417 Patented Dec. 31, 1968 chamber at this temperature until a crystal is formed, and a second position where the chamber is substantially removed from the bath so that it may be cooled rapidly.

By utilizing a bath of molten metal for heating the growing chamber to the proper temperature and holding the chamber at this temperature, the chamber may be rapidly heated and cooled by insertion and removal of the chamber with respect to the molten bath. In addition, the molten bath may be maintained at a substantially constant temperature. This temperature does not change rapidly because substantial heat energy is required to change the temperature of the metal. All of these advantages obtained by using a molten bath substantially improve the uniformity of the crystal growing process and reduce the overall time for producing each batch of crystals. This is a substantial advance over known apparatus for this general purpose.

In accordance with another aspect of the present invention, there is provided a method of growing a crystal formed from a vapor of crystal producing material. This method comprises placing a substrate seed in a refractory chamber at a given position; providing a bath of molten metal; maintaining the bath at a selected temperature; bringing the bath around the chamber and substantially above the given position; introducing vapor of the crystal producing material into the chamber; maintaining the bath around the chamber until the heat of the bath causes the vapor to grow a crystal onto the seed; and, then removing the molten metal bath from around the chamber at least in the area of the given position.

The above defined method results in a uniform crystal which can be produced in a lesser time than obtainable by other processes producing like crystals.

The primary object of the present invention is the provision of a method and apparatus for growing a crystal from a heated vapor of crystal producing material, which method and apparatus maintain the vapor temperature uniform during the growing process.

Another object of the present invention is the provision of a method and apparatus for growing a crystal from a heated vapor of crystal producing material, which method and apparatus involve a shorter time for heating and cooling the vapor between successive crystal growing processes.

Still another object of the present invention is the provision of a method and apparatus for growing a crystal from a heated vapor of crystal producing material, which method and apparatus use a bath of molten metal to heat the vapor and hold the vapor at the desired growing temperature.

These and other objects and advantages will become apparent from the following description used to illustrate the preferred embodiment of the present invention as read in connection with the accompanying drawing in which the sole figure is a cross-sectional, side elevational view showing, somewhat schematically, the preferred embodiment of the present invention.

Referring now to the drawing wherein the showing is for the purpose of illustrating the preferred embodiment of the invention only and not for the purpose of limiting same, the figure illustrates an apparatus A including a crystal growing chamber 10 formed from quartz, or similar material. The chamber includes a bottom wall 12, side walls 14 and a top flange 16 with the top flange being secured onto a cover 20 by a number of circumferentially spaced clamps 22. The cover 20 is in turn secured onto four support rods 30, 32, two of which are shown, which coact with bushings 34, 36 within a stationary or fixed plate 38. A vertically extending nonrotatable screw 40 is secured onto plate 20 by a nipple 42 so that a drive unit 44, driven by motor 46 through a shaft 48, can move the cover 20 vertically to control the 3 vertical position of the crystal growing chamber with respect to the fixed plate 38. The support rods 30, 32 guide the chamber as the screw is forced upwardly or downwardly by unit 44.

The unit 44 is actuated by a control mechanism energized by power supply lines L1, L2 through a switch 52. Above the screw 40 there are provided vertically spaced limit switches 60, 62 secured onto an electrically grounded plate 64. These limit switches control the continuity between lines 66, 68 and the ground connection 70 of control mechanism 50. A dial 72 on control mechanism 50 may be moved between a down or an up position. The upper portion of screw 40 is provided with a cam 74 which coacts with the limit switches 60, 62 to control the vertical position of the chamber 10 when dial 72 is in a given position. For instance, when the dial is moved to the down position, as show in the figure, unit 44 drives screw 40 downwardly until cam 74 opens limit switch 60. At this time the control mechanism 50 de-energizes motor 46. When dial 72 is placed in the up position, unit 44 drives the screw 40 upwardly until cam 74 opens limit switch 62. This de-energizes motor 46 and determines the upper position of chamber 10. Of course, a variety of other structures could be used to move the chamber 10 between an uppermost and a lowermost position, and the structure shown is only representative.

Referring now more particularly to the interior portion of chamber 10, there are provided a plurality of vapor inlets 80, 82 connected, respectively, with lines 84, 86. These lines direct a vapor or gaseous material from supply 88 to the interior of the chamber 10. Within the chamber there is also provided a baffle plate 90 supported on rod 92 so that the vapor flowing into the chamber is forced outwardly before it progresses downwardly into the lower portion of chamber 10. An outlet 94 is connected onto both a line 96 and a tube 98. After the vapor has been used within chamber 10, it is removed through the tube 98 so that additional vapor can be supplied to the chamber through the inlets 80, 82.

In the formation of crystals within chamber 10, substrate seeds 100 are positioned within the chamber on the lower wall 12. In a manner to be hereinafter described, the vapor issuing through inlets 80, 82 is heated to or held at a relatively high temperature so that the vapor is deposited onto the seeds to form solid state conductive devices. The reactive gas supply 88 includes means for heating a crystal producing substance to the vaporization temperature so that it may enter the chamber 10. In most instances, the vaporization temperature may be exceeded in supply 88 so that the temperature in this unit is greater than the growing temperature in chamber 10. The particular temperature relationship between vaporization and crystal growth is well known in the art.

The present invention is directed toward the apparatus and method for raising and/or maintaining the temperature of the vapor within chamber 10 at the desired growing temperature. In accordance with the preferred embodiment of the invention, there is provided a container having a bath 112 of molten metal extending to an upper level 114. The bath 112 can be formed from a variety of metals; however, the desired growing temperature for the vapor within chamber 10 dictates the temperature and in some respects the metal used within the bath. Some metals which may be used are lead, copper, bismuth and aluminum. A wide variety of other metals may also be used, as long as they have a melting point below the temperature which is to be maintained within the bath.

To melt the molten metal bath 112 and maintain it at a selected temperature, there is provided multiturn inductor surrounding container 110 and having an internal coolant passageway 122. Coolant, such as water, is passed through the inductor 120 so that it does not become overheated during use of apparatus A. The

inductor is supported with respect to container. 110 by a refractory casing 130, which is similar to the casing used in induction melting apparatus. A power supply, schematically represented as generator 132, is electrically connected across the inductor 120'. It is within the contemplation of the present invention to employ various frequencies for energizing the inductor 120. It has been found that frequencies between line frequency and radio frequency are well adapted for heating the metal bath to a given temperature and holding the bath at this temperature.

Since the heat dissipated from the bath is gradual, the convection and/or conduction through the bath maintains the bath at a somewhat constant temperature. This uniformiy is aided by heating the bath throughout is length with the coextensive inductor 120. A switch 134 selectively energizes the inductor 120. This switch is used during the initial operation of the apparatus A. Continuous control of the temperature within the bath is accomplished by selectively controlling current flow through the field winding 136 of generator 132.

A control unit 140, including a thermocouple 142 extending into bath 112, is used to control automatically the current fiow through the field winding 136. A control line 144 forms an electrical series circuit between the ground connections 1 46, 148. This series circuit includes the field winding 136. When the temperature of the metal within bath 112 is increased slightly, the thermocouple 142 opens the series circuit and de-energizes the generator 132. In like manner, when the temperature of bath 112 decreases slightly, the thermocouple 142 establishes the series circuit through the winding 136. This energizes the inductor 120 to again inductively heat the metal within bath 112. Since the bath changes temperature very gradually, the convection and stirring caused by the magnetic fields used in heating the bath maintain a somewhat constant temperature throughout the bath. This constant temperature is determined by the desired temperature which is to be used for effecting grain growth on the seeds 100 within chamber 10. As stated before, this temperature may be less than the temperature used to produce the reactive gas or vapor.

To somewhat inhibit the loss of heat from the molten bath 112 and prevent undue oxidation of the metal within the bath, a seal is provided between chamber 10 and container 110. Gaseous material is introduced beneath the seal through inlet 152. This gaseous material is exhausted through outlet 154 at the opposite side of the container. A line 156 communicates supply 158 of gaseous atmosphere creating material with the inlet 152. In like manner, a conduit 160 directs the exhausted atmosphere to an appropriate reservoir.

In operation, the molten bath 112 is raised to the desired temperature which is compatible with rapid crystal growth to take place within the chamber 10. Of course, the higher tthe temperature the more rapid the crystal growth. This feature must be balanced against the disadvantages of high temperature crystal growth. For instance, certain impurities which may be desired within the seeds 100 migrate rapidly from the seeds at relatively high temperatures. Also, at high temperatures the vapor within the chamber may not maintain a selected composition to impart the desired characteristics to the resulting crystal. For this reason, substantial technical information is available for determining the proper crystal growth temperatures when various materials are being used and various characteristics are desired.

After the desired temperature has been imparted to bath 112, the seeds 100 are positioned within the chamber 10 and the chamber is closed by clamps 22. Then the dial 72 is set to the down position. Unit 44 drives the chamber 10 downwardly to a proper position within the molten bath 112. The molten bath rapidly raises the temperature of the vapor within the container, and this temperature is held substantially constant by the action of control unit 140. After a predetermined time, which is determined by the material being crystallized and the desired crystal size, the dial 72 is moved manually or automatically to the up position. Unit 44 then pulls screw 40 upwardly so that the chamber is withdrawn from embodiments may be used without departing from the heating unit, the chamber cools rapidly. Crystals are then removed from the container, and new seeds 100 are inserted. The process is repeated to produce additional crystals.

Although a variety of crystal growing substances can be used, in accordance with one embodiment, silicon tetrachloride is used as the crystal forming vapor. When using this material, the bath 112 is heated between 850 C. and 1400 C. Preferably, the bath is maintained at 1200 C. during the crystal growth operation.

The present invention has been described in connection with one structural embodiment; however, various other embodiments may be used wtihout departing from the intended spirit and scope of the present invention as defined in the appended claims.

Having thus described my invention, I claim:

1. A method of growing a crystal formed from a vapor of crystal producing material, said method comprising the following steps:

(a) placing a substrate seed in a refractory chamber at a giveri position;

(-b) providing a bath of molten metal;

(c) maintaining said bath at a selected temperature;

(d) bringing said bath around said chamber and substantially above said given position;

(e) introducing vapor of said crystal producing material into said chamber;

(f) maintaining said bath around said chamber until the heat of said bath causes said vapor to grow a crystal onto said seed; and,

(g) then removing said molten metal bath from around said chamber at least in the area of said given position.

2. In an apparatus for growing a crystal from a vapor of crystal producing material, said apparatus comprising, a refractory chamber, means for introducing s-aid vapor into said chamber and means for controlling the temperature within said chamber to cause said vapor to form a crystal, the improvement comprising: said temperature control means including a container having a bath of molten metal, means for maintaining said bath at the desired temperature, and means for moving said chamber between a first position where said chamber is in said bath to heat said chamber to the crystallizing temperature and hold said chamber at said temperature until a crystal is formed and a second position where the chamber is substantially removed from said bath so that the chamber may be cooled after said crystal is formed.

3. The improvement as defined in claim 2 wherein said temperature maintaining means is an induction heating coil surrounding said bath, power means for energizing said coil and a thermostat device for controlling said power means to maintain said bath of molten metal at a substantially constant temperature.

4. The improvement as defined in claim 2 wherein said container includes a seal around said chamber and above said molten metal bath and conduit means for introducing a protective atmosphere between said seal and said bath.

5. The improvement as defined in claim 2 wherein said coil is encased within a refractory shell surrounding said container.

References Cited UNITED STATES PATENTS 3,243,323 3/19-66 Corrigan et a1. 117-106 3,341,360 9/1967 Nickl 117-201 ALFRED L. LEAVITT, Primary Examiner. A. GOLIAN, Assistant Examiner.

U.S. Cl. X.R. 

